widget: Emit initial style-set signal in the same situations as GTK2

widget: Emit initial style-set signal in the same situations as GTK2

This ensures that widgets that aren't ported and rely on the style-set
signal being emitted work as well as before. They should not rely on
style-set being emitted however.

Note that this function is a no-op if the initial style has been set
already and is very cheap if it has not been set yet. It only becomes
relevant if the resulting style actually gets used.

https://bugzilla.gnome.org/show_bug.cgi?id=639584